Only languages listed in the LANGUAGES setting can be selected. If you want to restrict the language selection to a subset of provided languages (because your application doesn’t provide all those languages), set LANGUAGES to a list of languages. For example:

A bit above there's another important part that explains HOW Django selects the language presented to the user. So, if you set LANGUAGE_CODE to "de" but your visitor (or yourself) send "en" as a requested language Django will answer in English.

So, that is already documented, you just need to read the whole document and interpret it, you'll see that LANGUAGE_CODE is the *last* thing that Django cares about when selecting the language.